This invention relates to vibrating systems and to an energy redistribution system for use in the vibrating system that functions to level the power spectral density of the vibrations and to redistribute vibrational energy among the three orthogonal linear and three rotational axes of vibration in a controllable manner.
It is a problem in the field of vibrating systems to control the amplitude of vibrations as well as the spectral density of vibrations in each of the three orthogonal linear and three rotational axes of vibration. One such vibrating system consists of a mass of predefined shape and extent that responds to a series of impulses by vibrating as a function of the characteristics of the mass, as well as the frequency, pulse shape and magnitude of the impulses applied to the mass and the location of the site at which the impulses are imparted. These vibrating systems typically have very irregular spectrum in the above-noted six axes of vibration.
In the field of vibrating systems, there are impact driven vibration test systems that are used to test products to determine if design or process defects are present.
In these vibration test systems, it is desirable to excite all (or at least selected) frequencies of vibration as well as all six axes of vibration (three translation axes and three rotation axes) simultaneously and in a controllable manner. The present impact driven vibration test systems available for the vibration excitation of products lack reasonably flat spectra in the various axes and the balance among these axes (the overall level of vibration in each axis) is usually not very uniform. Exact uniformity is not required, but some commercially available impact driven vibration test systems have spectral density variations that leave some frequencies and axes of vibration essentially unexcited and some axes and frequencies of vibration excited to comparatively overly high levels. Additionally, the impact driven vibration test systems are usually very weak in the critical low frequency areas of vibration. The existing impact driven vibration test systems not only suffer from a lack of vibration in some frequencies and axes of vibration but also do not have any apparatus that allows the translation of energy from high frequencies to low frequencies of vibration.
The impact driven vibration test systems are typically implemented as a basic shaker table that includes a platform upon which the product is mounted. The platform is supported on flexible supports that permit the vibration of the table freely in all directions, independent of the environment. The shaker table generates vibration in six axes by providing either pneumatically driven or mechanically driven actuators, termed exciters or vibrators, that produce an impact to initiate the vibrations. The platform couples the vibrations from the actuators to the product. The typical actuator is an impact device that produces forces of high magnitude but very short duration, typically driven by air pressure. There are two effects that result from this input: the repeated impacts generate a line spectrum (equally spaced lines) in the spectral density domain, the shaker table is set into a quasi-resonance condition and all of its modes of vibration are excited. As a result, the spectral density of the shakers is not uniform and can vary by six or more decades. These variations are unacceptable for highly accelerated testing or for simulation applications.
The physical properties of the shaker table components cause the shaker table to respond to the different frequencies in the impact spectrum in different ways. The physical properties of the shaker table components typically resonate with certain vibration frequencies and suppress other vibration frequencies to result in selected modes of vibration. For example, resonation results in the vibrations remaining for a relatively long time compared to the duration of the input pulse, while suppression results in the quenching of the vibration in a relatively short time. The modes of vibration of the shaker table which are excited are also a function of the location, orientation and nature of the actuators as well as the dimensions and properties of the platform. Thus, by designing the shaker table to have relatively low resonant frequencies, the spectral response of the system can be shifted to fill up the low frequency end of the spectrum, but there is a tendency to have significant variation in spectral density.
This shaker table architecture is well known and the great difficulty facing the test engineers in this field is the implementation of the shaker table in a manner to precisely produce the desired vibration conditions in terms of the presence of selected vibration frequencies and regulation of their magnitude. There are obviously numerous variables, each of which affects the magnitude and frequency of the vibrations that are produced. These variables include but are not limited to: number of actuators, actuator placement, actuator characteristics, frequency of actuator operation, physical coupling of the actuator to the shaker table platform, coupling of the product to the shaker table platform, damping elements included in the shaker table, dimensions of the shakertable, shakertable implementation, including materials and intervening structures. A further complicating factor is that these variables can also be interactive, in that the variation of one variable can impact the effects produced by another variable. Thus, the design of a shaker table is a non-trivial task and typically represents a compromise that produces a crude emulation of the desired vibration characteristics. The quest for accuracy in this field is ongoing and has been relatively unsuccessful to date.
Thus, while there exist numerous variations of shaker tables, each implementation presents limitations that prevent the test engineer from effecting precise control over the vibration frequencies and magnitudes to thereby precisely emulate the environment that the product under test will encounter or the environment desired for simulation or stimulation.
The above-noted patent application titled: xe2x80x9cTuned Energy Redistribution System for Vibrating Systemsxe2x80x9d addresses this problem by selectively reducing discrete frequencies of vibration and translating them into frequency limited broad band vibrations. However, the system described in this patent application is limited in terms of not addressing the simultaneous redistribution of vibration among the six axes of vibration (three translation axes and three rotation axes).
U.S. Pat. No. 4,164,151 issued Aug. 14, 1979 to Douglas C. Nolan et al discloses a random vibration generator that includes a hollow table top for supporting equipment to be subjected to vibration and a sinusoidal reaction-type vibration machine connected to the tabletop to produce a sinusoidal vibration of adjustable frequency and amplitude. The hollow tabletop is divided into four sections, each containing a number of projectiles, such as heavy balls which roll and bounce around within these four sections, impacting the floor and ceiling of the four sections and each other in random fashion to produce random shocks over a wide band of frequency and amplitude, thereby subjecting equipment connected to the tabletop with every possible vibration failure mode that might occur in nature.
Therefore, the field of vibrating systems is devoid of any apparatus that is operable to controllably redistribute energy from axes and frequencies of high acceleration to axes and frequencies of low acceleration. Additionally, the impact driven vibration test systems are usually very weak in the critical low frequency areas of vibration. The existing impact driven vibration test systems not only suffer from a lack of vibration in some frequencies and axes of vibration but also do not have any apparatus that allows the translation of energy from high frequencies to low frequencies of vibration.
The above-described problems are solved and a technical advance achieved by the present energy redistribution system for a vibrating system that is operable to redistribute energy from axes and frequencies of high acceleration to axes and frequencies of low acceleration. The present energy redistribution system for a vibrating system enables one to balance the vibration spectra and over all levels between axes. In addition, the energy redistribution system for a vibrating system can be used to enhance the low frequency content of the spectra by redistributing energy from the existing vibrations in the high frequency region of the spectrum.
The energy redistribution is accomplished by placing a shaped object into an enclosure, termed an equalizing module, which is attached to the vibrating system. The location of the equalizing module affects the frequency and direction from which energy is removed as the shaped object responds to the vibration present at the location of mounting of the equalizing module. For example, if the location of the equalizing module is at a node (point of no displacement) of a particular mode of the shaker table, then no energy is extracted from that mode at that particular mounting position. If the equalizing module is placed at an anti-node (point of maximum displacement), then the shaped object experiences significant inputs due to the vibration of that mode. When the shaped object impacts an interior surface of the equalizing module, an elastic, or nearly elastic, collision occurs. Since the mass of the shaped object is much less than the mass of the interior surface of the equalizing module that is impacted, the shaped object behaves nearly as if it has impacted an infinite mass. If the interior surface of the equalizing module is moving away from the shaped object, the shaped object rebounds with less velocity than it had before impact. The relative velocity, however, is the same from impulse-momentum theory. If the interior surface of the equalizing module is moving toward the shaped object at the instant of impact, the interior surface of the equalizing module imparts additional velocity to the shaped object. The shaped object flies away in either case to impact another interior surface of the equalizing module. The initial impact causes an infinite series of line spectra to be generated in the table (normal to the impact) with a cut off frequency (frequency where the spectrum rolls off) as determined by the programmer or flexible interior surface of the equalizing module as described below. This impact event can translate high frequency vibration into low frequency vibration in the table if a soft programmer is used. The use of programmers to shape the impact pulses allows the translation of vibration from one frequency and axis of high acceleration to a broad band of frequencies with the upper cutoff frequency defined by the programmer stiffness. The programmer stiffness can be different in each axis or on each plane of the impact surfaces, and shapes the vibration translated from one plane to another.
It is normal for the vertical acceleration of the impact type of shakers to be substantially more than in the horizontal directions. This is so because the shakers are somewhat like plates and bend out of plane much more than they distort in the plane of the plates. This being the case, the equalizing module takes energy from the vertical direction and transfers it to the horizontal directions.
The shaped object can be coated with a programmer material, that is, with a plastic like material, which shapes the shock pulses when the shaped object impacts the interior surfaces of the equalizing module. This impact imparts momentum to the shaped object with a nearly elastic impact. The shaped object then flies off and impacts another interior surface of the equalizing module and provides an impulse, shaped by the programmer material, that generates a line spectrum in the acceleration spectral density domain. Since the impact is a single event, the line spectrum contains an infinite number of lines up to the cut off frequency determined by the stiffness of the programmer. A short pulse generates a very broad spectrum and a long pulse generates a narrow spectrum. Both result in the low frequency portion of the spectrum and roll off at the higher frequencies as learned from Fourier theory.
The interior surface of the equalizing module can be also made to be elastic, that is, very springy or flexible, and to bend under the impact. This acts as a programmer and shapes the shock pulse. Note that various walls can have different spring rates and shape the spectra differently. Therefore, an axis that is required to have more low frequency acceleration can receive it by making the interior surface of the equalizing module that is normal to that axis flexible. From Newton""s law on action and reaction, the shaped object carries only one thing and that is a vector momentum from the last collision and rebound. The interior surfaces of the equalizing module having the highest velocity transfers the most momentum to the shaped object and therefore transmits more momentum to another axis. It is noted that the interior surfaces of the equalizing module all move in a random manner over a very broad frequency range and that calculating the motion of the shaped object is essentially impossible without performing a computer simulation of a specific shaker table with specific vibrator locations.
The present energy redistribution system for a vibrating system has advances over the existing impact driven vibration test systems in that the present energy redistribution system for a vibrating system:
1. Redistributes vibrational energy over all axes of vibration, not just the vertical axis.
2. Every interior surface of the equalizing module can have a different stiffness or be coated with a different programmer material. Additionally, the shaped object can be coated with an elastomer which acts as a programmer.
3. The mounting location of the present energy redistribution system for a vibrating system can select or reject modal frequencies and axes to be reduced or enhanced.
4. The equalizing module can be any shape, not just a rectangular one.
Therefore, the present energy redistribution system for a vibrating system is operable to redistribute energy from axes and frequencies of high acceleration to axes and frequencies of low acceleration. The present energy redistribution system for a vibrating system enables one to balance the vibration spectra and over all levels between axes. In addition, the energy redistribution system for a vibrating system can be used to enhance the low frequency content of the spectra by redistributing energy from the existing vibrations in the high frequency region of the spectrum.